1. Field of the Invention
The present invention relates to a stratified scavenging two-stroke engine of an air-leading type, which uses pre-scavenging by air.
2. Description of Related Art
In a stratified scavenging two-stroke engine of an air-leading type, during an upward stroke of a piston, a gaseous mixture is introduced from an intake passage into a crankcase and an air is introduced from an air passage into a scavenging passage by a negative pressure formed in the crankcase. Further, during a downward stroke of the piston, the air that has been introduced into the scavenging passage during the upward stroke is introduced into the cylinder as an air for pre-scavenging prior to introduction of the gaseous mixture from the crankcase into the cylinder. Consequently, since a layer of air is present between a combustion gas of the gaseous mixture and a gaseous mixture that is newly supplied through the scavenging passage, it is possible to prevent the gaseous mixture from being mixed into the combustion gas, and to thereby prevent blow-by of an unburned gas through an exhaust passage. WO2010/035684 publication discloses an example of this type of stratified scavenging two-stroke engine.
In addition, such a stratified scavenging two-stroke engine is employed as an engine of a portable working machine (including a handheld working machine and a backpack working machine) such as a chainsaw. In recent years, downsizing of such a portable working machine is increasingly required, and along with this requirement, downsizing of a stratified scavenging two-stroke engine being an engine of such a working machine is also required. Further, from the viewpoint of reduction of environmental load, further improvement in cleanness of exhaust gas is also required.
Under the circumstances, it is an object of the present invention to provide a stratified scavenging two-stroke engine having a compact construction. Further, it is another object of the present invention to provide a stratified scavenging two-stroke engine, which although it has a compact construction, which can obtain a greater amount of air for pre-scavenging, and which can suppress blow-by of unburned gas.